1. Field of the Invention
The present invention relates to a process for preparing slightly soluble silver salt grains and, more particularly, to a process for preparing slightly soluble silver salt grains by controlling the electrode potential of the reaction solution containing an organic liquid.
2. Description of the Prior Art
Slightly soluble silver salt grains, or silver salt grains insoluble in a reaction solvent or a coating solvent, are often used in the photographic field. Typical examples of such grains are light-sensitive silver halide grains. These silver halide grains are usually prepared by reacting silver nitrate with a halogen ion providing agent in a hydrophilic colloid solution. It is known to change the crystal habit or the particle size distribution of the resulting silver halide grains by controlling, during the reaction, the electrode potential of the reaction solution, for example, the pH or pAg (silver ion concentration), e.g., as disclosed in C. E. K. Mees & T. H. James, The Theory of the Photographic Process, 3rd Ed., Chap. 2, Macmillan (1976). Mees et al., supra, discloses that octahedral silver halide grains are obtained with a high pAg value and cubic silver halide grains are obtained with a low pAg value, and that by maintaining the pAg constant the particle size distribution of the resulting silver halide grains can be narrowed, and generation of fog can be prevented by maintaining the pH constant.
A means for measuring the electrode potential of the reaction solution is necessary for controlling the electrode potential of the reaction solution. In preparing light-sensitive silver halides in the hydrophilic colloid solution as described above, a half cell combination is used wherein a calomel electrode or a silver/silver chloride electrode is used as a reference electrode and a glass electrode or a silver electrode is used as a measuring electrode.
However, since these reference electrodes use a potassium chloride aqueous solution or a hydrochloric acid aqueous solution as an internal solution, halogen ions contained in the internal solution react with the silver ion contained in a solution to be measured or a reaction solution, and tend to adversely affect the measured electrode potential.
Grains of silver salts of organic compounds containing an imino group, a mercapto or a thione group, or a carboxyl group (hereinafter referred to as organic silver salt grains) described in U.S. Pat. Nos. 3,094,417, 3,152,904, 3,457,075, Japanese Patent application (OPI) No. 36,020/77 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application"), etc., are illustrative of another type of slightly soluble silver salts to be used in a different photographic field, i.e., in the field of heat-sensitive materials or thermally developable light-sensitive materials. Many of these organic silver salt grains are prepared by dissolving organic silver salt-forming agents (i.e., above-described organic compounds or the non-silver salts thereof) in an organic liquid, adding water thereto to form an oil-in-water emulsion, and reacting the resultant emulsion with an aqueous solution of a silver ion-providing agent such as silver nitrate or a silver-ammine complex salt. Control of the pH or the pAg of the reaction solution during preparation of these organic silver salt grains for the purpose of changing the particle size distribution or the particle form of the grains is not known. However, it might be highly expected that, when the pAg is maintained constant, the particle size distribution of the resulting grains would be narrowed and, when the pH is maintained constant, fog could be prevented. Even if the electrode potential of the reaction solution for producing the organic silver salt grains using the above-described calomel electrode or silver/silver chloride electrode were to be attempted, accurate measurement of the electrode potential would be impossible since silver ion in the reaction solution reacts with halide ion in the internal solution of the reference electrode.
It is known to prepare slightly soluble silver salt particles such as silver halide grains or organic silver salt grains in an organic liquid medium. For example, U.S. Pat. No. 3,713,833 discloses a process of preparing light-sensitive silver halide grains or organic silver salt grains in an organic liquid solution of a vinyl copolymer. Also, U.S. Pat. No. 3,700,458 describes a process for preparing heavy metal salts of organic carboxylic acids in a non-aqueous solvent. However, conducting the reaction while controlling the pH or the pAg of the reaction solution during preparation of the slightly soluble silver salt grains using an organic liquid as a reaction medium is not known. Control of the pH or the pAg cannot be attained by using the above-described calomel electrode or silver/silver chloride electrode as a reference electrode. Because, there is the problem that accurate control of the electrode potential is impossible since the liquid (junction) potential difference between a solution to be measured and an internal solution of the reference electrode increases because the two solutions and different and the indicated electrode potential fluctuates.